This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
With the evolution of Long Term Evolution (LTE) system, LTE network switches from a homogeneous network into a heterogeneous network, where Macro eNode-Bs (eNBs) have higher transmission power for coverage purpose and pico eNBs have lower transmission power for capacity purpose. As verified, the handover failure rate is increased in such a heterogeneous network. It is therefore proposed that UE is connected to both a Macro eNB and a pico eNB concurrently, which is called dual connectivity, as shown in FIG. 1.
Dual connectivity is a feature defined from the UE's perspective, where a UE may simultaneously receive from and transmit to at least two different network points as shown in FIG. 1. Dual connectivity is one of the features that are being standardized within the umbrella work of small cell enhancements within 3GPP Rel-12.
Dual connectivity is defined for the case when the aggregated network points operate on the same or separate frequency. Each network point that the UE is aggregating may define a stand-alone cell or it may not define a stand-alone cell. It is further foreseen that from the UE's perspective, the UE may apply some form of Time Division Multiplexing (TDM) scheme between the different network points that the UE is aggregating. This implies that the communication on the physical layer to and from the different aggregated network points may not be truly simultaneous.
Dual connectivity as a feature bears many similarities with carrier aggregation and Coordinated Multi Point Transmission/Reception (CoMP). The main differentiating factor is that dual connectivity is designed by considering a relaxed backhaul and less stringent requirements on synchronization requirements between the network points. This is in contrast to carrier aggregation and CoMP, where tight synchronization and a low-delay backhaul are assumed between connected network points.
Due to complicacy, some UEs support dual connectivity at Layer 2 and Layer 3. In other word, their physical layer can only connect with either Macro eNB or pico eNB at the same time slot. In order for this type of UE to work in dual connectivity scenario, subframes have to be split into two sets, subframes within one set are used for communications between UE and Macro eNB, subframes within the other set are used for communications between UE and pico eNB.
There are two different design possibilities of dual connectivity which are closely related to UE capabilities. There are different design challenges in the two design options and some tradeoff between UE complexity and system design effort is also envisioned.
For UEs capable of simultaneously transmitting to (receiving from) the dual connected nodes, there is much less constraint on system design. However, it would require UL dual carrier support in a separate carrier deployment scenario. In a same carrier deployment scenario, there will be some “dead zones” where the UE cannot hear from the two nodes simultaneously due to the large difference in dynamic range, e.g. when the UE is very close to the Pico. Moreover, there will be problems with inter-modulation products in UL when superpositioning two signals.
For UEs incapable of simultaneously transmitting (receiving) to (from) the dual connected nodes, a TDM-based dual-connectivity scheme is needed. Some semi-static resource partitioning between the two nodes is needed for continuous connection. The system design will become more complex than the previous scheme. But from the UE's perspective, the implementation complexity is reduced. For example, the UE does not need to monitor scheduling grants from the two nodes. However, the UE may need some guard time to switch connections which may reduce the spectrum efficiency.
The present disclosure will focus on the TDM-based dual-connectivity scheme. It is worth noting that TDM can be either in DownLink (DL) or in UpLink (UL), or even in both links. Furthermore, it is applicable to both TDD and FDD.
To fulfill the dual-connectivity target in a TDM manner, the subframes need to be partitioned into multiple subsets, each used for communications with one node. Such a subframe partitioning is indeed subjected to certain crucial restrictions. One example restriction is the 8 ms periodicity of the HARQ timing in FDD.
FIG. 2 illustrates scheduling and HARQ timing for UL transmission (FDD). As illustrated in FIG. 2, an initial UL transmission in subframe #n needs to be acknowledged in the DL in subframe #n+4, and an UL retransmission needs to be initiated in subframe #n+8, if initial transmission fails. Keeping this HARQ timing in mind, a straightforward scheme for partitioning subframes is to group subframes [#0, #8, #16, . . . ] in the DL and subframes [#4, #12, #20, . . . ] as one subframe set, and then group the remaining subframes as another subframe set:                Subframe set #0:                    DL [#0, #8, #16, . . . ] and            UL [#4, #12, #20, . . . ]                        Subframe set #1:                    DL [#1˜#7, #9˜#15, . . . ] and            UL [#0˜3, #5˜#11, #13˜#19, . . . ]                        
For sake of display accuracy, FIG. 2 only shows fourteen subframes. However, it will be appreciated that FIG. 2 involves subframes with higher numbers than the illustrated subframes.
Subframe set #0 can be used to communicate with the anchor node (for control-plane maintenance and small amount user-plane traffic), while subframe set #1 used by the booster carrier (for traffic offloading and huge capacity improvement).
The situation in TDD is similar, although more restrictions need to be considered in the partitioning of subframes due to various UL/DL configurations.
Consequently, feasible subframe partitioning is subjected to many restrictions, and thus, only certain patterns (e.g., periodicity) can be used to maintain dual-connectivity in a TDM manner.